The present invention relates to a liquid crystal display device used as a flat display for OA equipment and the like.
In recent years, a liquid crystal display device (hereinafter LCD) has been widely used as a flat display in personal computers, word processors, car navigation systems and the like because of such features as thin construction, light weight and small power consumption. Furthermore, hopes run high nowadays that an LCD will replace a CRT display, resulting in a vigorous effort to develop and commercialize a large screen LCD . As the screen size of an LCD increases, a much more requirement has been made of the LCD to increase viewing angles.
A few methods have so far been proposed to realize an increase in viewing angle of an LCD . Among the methods proposed, a method utilizing an OCB (Optically Compensated Birefringence) mode, in which liquid crystal molecules are bend-aligned, receives much attention even as a display device for moving pictures since the switching speed thereof shows the order of several milliseconds, which is faster than the speed of a normal TN mode by an order of magnitude.
The liquid crystal molecules used in the OCB mode shows optically positive uniaxial anisotropy in refractive index, together with configuration anisotropy and dielectric anisotropy. The optical axis of the liquid crystal molecules coincides with the direction of the molecules"" major axis. The bend alignment means that within a liquid crystal layer 1 sandwiched between two substrates 3u and 3b, as FIG. 11 shows a cross-sectional view thereof on an X-Z plane, liquid crystal molecules 20 are aligned within the X-Z plane from the top to downward while changing respective alignment directions in such a way as they are aligned nearly symmetrically with respect to a plane 21 that divides the liquid crystal layer 1 into equal two halves. In the OCB mode, the effective retardation of a liquid crystal layer can be varied by changing the direction of liquid crystal molecules through a voltage applied to liquid crystal cells. When a change in retardation takes place, the polarization state of the light passing through the liquid crystal layer changes. This change in the polarization state is picked up as a change in transmissivity by the action of an analyzer and a polarizer, and is used for image displaying.
In the Published Unexamined Japanese Patent Application No. H7-49509, disclosure is made on a technology for expanding a viewing angle range in addition to lowering a driving voltage by attaching a phase plate for producing a negative phase difference to the liquid crystal cells, which are bend-aligned as described above. Accordingly, the viewing angle range, where no inversion of black and white takes place, is expanded in comparison with the case where no phase plate is used. As a setup for producing a negative phase difference, such an arrangement as the optical axis of a positive birefringence medium being made perpendicular to the optical axes of liquid crystal molecules or as the optical axis of a negative birefringence medium being made parallel to the optical axes of liquid crystal molecules or the like is proposed. Also, as the material for producing a negative phase difference, a proposal is made, as the Published Unexamined Japanese Patent Application No. H8-327822 discloses, about a phase difference film having discotic liquid crystal molecules with negative refractive index anisotropy hybrid-aligned.
However, with the conventional LCD as described above, as the angle of a viewer""s line of sight making with the normal to the display surface increases, the brightness in the black display state observed on the display screen increases, thus indicating a problem that the contrast is still highly dependent on a viewing angle. Particularly, in the direction perpendicular to the optical axes of liquid crystal molecules sandwiched between substrates, the viewing angle dependency in the state of black displaying is large enough to cause a problem of substantial degrading of the contrast in a black displaying state.
The present invention deals with the foregoing problem and proposes an LCD with excellent viewing angle characteristics by preventing the contrast degradation in the direction perpendicular to the optical axes of liquid crystal molecules even in the black displaying state where the viewing angle dependency is especially large.
A liquid crystal display of the present invention has a structure comprising:
two substrates;
a liquid crystal layer sandwiched between the two substrates, in which liquid crystal molecules are bend-aligned;
a polarizer and an analyzer with respective polarization axes intersecting each other at right angles;
a uniaxial phase plate having negative optical anisotropy;
a uniaxial phase plate having positive optical anisotropy and having the optical axis thereof made perpendicular to the direction of orthogonal projection of the optical axes of liquid crystal molecules onto the substrate; and
an additional uniaxial phase plate having positive optical anisotropy and having the optical axis thereof made not perpendicular to the direction of orthogonal projection of the optical axes of liquid crystal molecules onto the substrate, and more preferably the direction of the optical axis of the additional uniaxial phase plate is made nearly parallel to the direction of the polarization axis of the analyzer. Accordingly, the liquid crystal display device is allowed to prevent the contrast from degrading in the direction perpendicular to liquid crystal molecules even in the black displaying state where a viewing angle dependency is large, thereby achieving excellent viewing angle characteristics.
Also, a liquid crystal display device of the present invention has a structure comprising:
two substrates;
a liquid crystal layer sandwiched between the two substrates, in which liquid crystal molecules are bend-aligned;
a polarizer and an analyzer with respective polarization axes intersecting each other at right angles;
a biaxial phase plate having positive and negative optical anisotropy; and
a uniaxial phase plate having positive optical anisotropy and having the optical axis thereof made not perpendicular to the direction of orthogonal projection of the optical axes of liquid crystal molecules onto the substrate.
Also, a liquid crystal display device of the present invention has a structure comprising:
two substrates;
a liquid crystal layer sandwiched between the two substrates, in which liquid crystal molecules are bend-aligned;
a polarizer and an analyzer with respective polarization axes intersecting each other at right angles;
a phase plate formed of an optical medium having negative refractive index anisotropy with the major axis thereof being hybrid-aligned;
a uniaxial phase plate having positive optical anisotropy; and
an additional uniaxial phase plate having positive optical anisotropy and having the optical axis thereof made not perpendicular to the direction of orthogonal projection of the optical axes of liquid crystal molecules onto the substrate.
As described in the above, by having an additional uniaxial phase plate with positive optical anisotropy disposed so as to make the major axis thereof not perpendicular to the direction of orthogonal projection of the optical axes of liquid crystal molecules onto the substrate and, in particular, to make nearly parallel to the direction of the polarization axis of the analyzer, the liquid crystal display device is allowed to achieve a high contrast over a very wide range of viewing angle.